To control the sensitivity of a balance, it is an established procedure that from time to time an accurate calibration weight is put on the balance and weighed, and that correction factors are determined based on the weighing result. The known state of the art also includes balances that have a built-in calibration weight which, when necessary, can be coupled to the load-receiving part of the weighing mechanism in the manner of a weight that is operated by a dial selector mechanism.
If the calibration weights are put directly on the load-receiving part, the mass as well as the volume of the calibration weights will correspond directly to the capacity load of the balance and will therefore be large. The mass and volume of the calibration weight can be reduced if a lever mechanism is arranged between the calibration weight and the load-receiving part, whereby the force generated by the calibration weight is transferred to the load-receiving part through a lever reduction.
A balance of this type, which operates according to the principle of electromagnetic force compensation and has a built-in rod-shaped calibration weight, is disclosed in U.S. Pat. No. 6,194,672 B1 to Burkhard et al., which is owned by the applicant. As the calibration weight arm is permanently coupled to the load-receiving part, it only serves to receive the calibration weight during the calibration process and is not part of the calibration weight itself.
A weighing cell which likewise operates according to the principle of electromagnetic force compensation is disclosed in U.S. Pat. No. 6,232,567 B1 to Bonino et al., also owned by the applicant. It has a calibration weight receiver that is permanently screwed to the load-receiving part. The calibration weight receiver is configured so that it can receive a calibration weight from a non-disclosed calibration-weight-holding mechanism.
A calibration device for a weighing cell is disclosed in European patent application 0 020 030 A1. The weighing cell is of a state-of-the-art configuration and has a load-receiving part and a stationary part. A calibration weight arm is permanently connected to the load-receiving part. Two pairs of bolts are rigidly connected to the calibration weight arm and serve as calibration weight receivers for two weights which can be set onto the bolt pairs independently of each other, either each weight individually or both weights simultaneously, for the purpose of checking and calibrating the weighing cell. When the two weights are not seated on the bolt pairs to perform a calibration, they are secured by means of angled levers and abutments in a vertical support plate which is connected to the stationary part of the weighing cell through support elements.
A weighing cell with strain gauges is disclosed in commonly-owned U.S. Pat. No. 6,414,252 B1 to Emery et al., which has a cylindrical calibration weight on a U-shaped calibration weight arm. The calibration weight arm is part of a lever mechanism to transmit and amplify the calibration weight force acting on the load-receiving part of the weighing cell and remains rigidly connected to the load-receiving part also while the weighing is taking place.
Another weighing cell with strain gauges is disclosed in Japanese patent application 2002-168683 A, which has a calibration weight with a reduction lever, where the reduction lever and the calibration weight are used for calibration as well as for dead-weight compensation. Several block-shaped calibration weight pieces are rigidly connected to the reduction lever. The calibration weight and the reduction lever in this arrangement form a unit and, unlike the devices of the aforementioned patents and publications, they cannot be uncoupled from each other. By shifting the fulcrum point of the reduction lever by means of a lifting device, one of the two functions of calibrating or dead-weight compensation is selected. Thus, the dead-weight compensation weight or calibration weight remains constantly in interactive connection with the weighing cell.
Calibration weights, in particular the kind of calibration weights whose weight force is transmitted to the load-receiving part through a lever-reduction mechanism, need to take a defined, reproducible position in relation to the calibration weight arm on which the weight is set in the calibration process. The defined, reproducible position is achieved through appropriate positioning means that are located on the calibration weight and enter into engagement with a correspondingly configured position-defining seating area on the calibration weight arm. In most cases, the calibration weight is in addition provided with specially configured transfer means which also position the calibration weight in relation to a transfer mechanism which has the function of setting the calibration weight on the calibration weight arm and lifting it off. No wear or exchange of material can be allowed to take place at the places of contact between the calibration weight and the calibration weight arm, or between the calibration weight and the transfer mechanism. To prevent that a calibration weight gains mass over its useful lifetime, one uses materials whose surfaces will, as much as possible, not oxidize, or materials with a surface on which a dense, stable oxide layer forms spontaneously already during the time immediately after the machining of the piece and before it is being used.
The accuracy of a balance depends substantially on the load capacity and the resolution of the weighing cell. This has the consequence that model series of balances are developed with a stepwise progression of load ranges from one model type to the next. It is advantageous if the mass of the calibration weight is matched to the upper limit of the load range, i.e., to the capacity load of the balance, in order to keep the error of the correction value and, accordingly, the influence of this error on the weighing result as small as possible. Consequently, every balance type in the series has its own calibration weight, matched to the weighing cell and its capacity load. The size of the balance housing is in this case determined by the required design space. If the design objective calls for a series of balance types with different load ranges using the same housing, then the size of the housing is determined by the largest load range, particularly if weighing cells are used that are not equipped with a lever mechanism for the reduction of the calibration weight force. Using separate designs for different load ranges leads to a large number of different calibration weights and thus to small production numbers. Depending on the configuration of the calibration weights and the weighing cells that are being used, it will further be necessary to produce different calibration weight arms. In the interest of economy, one will therefore choose the simplest possible shapes for the calibration weights, for example cylindrical shapes, which are made of one piece in order to avoid additional costs for assembling them. In conclusion, the disadvantages of the calibration weights disclosed in the prior art are in the spatial requirements imposed by the need to choose a simple shape, and the requirement for a large diversity of calibration weights for use in different gravimetric measuring instruments, in particular different balance types in a design family or model series of balances.
At least the exemplary embodiments of the present invention therefore have the objective to provide a calibration weight of the type that can be set on a calibration weight arm with a design that allows the calibration weight to be used in a diversity of differently built gravimetric measuring instruments, in particular in the different balance types in a design family of balances.